| 311. |
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| 312. |
- Sjögren, Rickard, et al.
(författare)
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Electron contamination in clinical high energy photon beams
- 1996
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 23:11, s. 1873-1881
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Tidskriftsartikel (refereegranskat)abstract
- The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a ''clean'' reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.
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| 313. |
- Sjölin, Martin, 1987-, et al.
(författare)
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A method for geometric calibration of edge-on detectors in a CT-gantry
- 2016
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 43:11, s. 6165-6174
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Photon-counting edge-on detectors are currently being considered for usein clinical computed tomography (CT) systems. A method for geometric calibrationof edge-on detectors mounted in a CT gantry has been developed and evaluated.The method is complementary to the geometrical calibration methods developed forCT systems using flat-panel detectors and takes the extra dimension of the edge-ondetectors (along the direction of the x-rays) into account.Methods: The method uses projection images of a simple phantom together withgeometrical arguments to accurately estimate the orientation and relative position ofthe edge-on detectors. Both computer simulations and experimental measurementswere used to verify the method.Results: It is experimentally demonstrated that the method can determine the orientationof the detector with an accuracy of 0.08 degrees. The method is also shownto be insensitive to errors in the modeled parameters used in the algorithm.Conclusions: The presented method can accurately determine the orientation andrelative position of edge-on detectors mounted in a CT-gantry and can be used toevaluate the detector mounting and to produce an accurate forward model of theimaging system. Also, the method has potential to reduce the dimensionality of thegeometric calibration of the full CT system since the direction of the x-rays withrespect to the detector is measured.
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| 314. |
- Sjölin, Martin, et al.
(författare)
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Angular oversampling with temporally offset layers on multilayer detectors in computed tomography
- 2016
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Ingår i: Medical physics (Lancaster). - : AAPM - American Association of Physicists in Medicine. - 0094-2405. ; 43:6, s. 2877-2883
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Todays computed tomography (CT) scanners operate at an increasingly high rotation speed in order to reduce motion artifacts and to fulfill the requirements of dynamic acquisition, e.g., perfusion and cardiac imaging, with lower angular sampling rate as a consequence. In this paper, a simple method for obtaining angular oversampling when using multilayer detectors in continuous rotation CT is presented. Methods: By introducing temporal offsets between the measurement periods of the different layers on a multilayer detector, the angular sampling rate can be increased by a factor equal to the number of layers on the detector. The increased angular sampling rate reduces the risk of producing aliasing artifacts in the image. A simulation of a detector with two layers is performed to prove the concept. Results: The simulation study shows that aliasing artifacts from insufficient angular sampling are reduced by the proposed method. Specifically, when imaging a single point blurred by a 2D Gaussian kernel, the method is shown to reduce the strength of the aliasing artifacts by approximately an order of magnitude. Conclusions: The presented oversampling method is easy to implement in todays multilayer detectors and has the potential to reduce aliasing artifacts in the reconstructed images.
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| 315. |
- Sjölin, Martin, 1987-, et al.
(författare)
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Compression of CT sinogram data by decimation in the view direction
- 2017
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405. ; 44:9, s. E138-E146
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Tidskriftsartikel (refereegranskat)abstract
- Pupose: In clinical computed tomography (CT), the image data is acquired during continuous rotation. If the time during which the signal is integrated (the frame time) is too long, the data is blurred in the view direction (i.e., azimuthal blur). This can be overcome by having a high angular sampling rate, but for systems with limited bandwidth, the increased amount of data can be a problem. In this paper, we evaluate the benefit of maintaining a high angular sampling rate on the CT gantry and performing a decimation (digital low-pass filtration followed by a downsampling) in the view direction before the bottleneck of the data transfer chain. Methods: A theoretical evaluation of the effects of the decimation is presented and the implementation of the digital filter is discussed. The compression scheme is evaluated on image data of a CATPHAN (R) 504 phantom and a human skull phantom. Results: It is shown that digital decimation can be used to compress data before read-out with more remaining data fidelity compared to having longer frame times. Specifically, the method is shown to preserve the detail in the reconstruction of the CATPHAN resolution patterns and the human skull phantom. It is also demonstrated that the method can be used to prevent aliasing artifacts. Conclusions: Decimation in the view direction is presented as an alternative to increasing the frame time for CT systems with limited bandwidth of the data read-out. The method can be used to either remove aliasing artifacts or preserve spatial resolution. The proposed compression scheme can be implemented on the CT gantry and thus reduce the bandwidth requirements on the data transfer.
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| 316. |
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| 317. |
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| 318. |
- Stathakis, S, et al.
(författare)
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Monte Carlo Based Dose Verification for Serial Tomotherapy
- 2007
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Ingår i: Proceedings in 49th AAPM Annual Meeting, Minneapolis, Minnesota, USA, July 22-26, 2007. - : Wiley.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: To develop a Monte Carlo model to verify the final dose distributions and monitor units for serial tomotherapy plans developed and delivered using the Peacock system (Corvus Treatment Planning System and MIMiC collimator, Nomos Corp., Sewickley, PA). Materials and methods: The Peacock system delivers the dose to the patient using arc therapy. The treatment plan is created in Corvus were sinograms are created for each arc in order to dictate the state of each of the MIMiC leaves at different locations along the arc. In-house functions were written in Matlab (Mathworks Inc., Natick, MA) to decode these sinograms. A simple three-field plan (three gantry positions), as well as full patient treatment plans were simulated using our Monte Carlo model and the same plans were delivered using the Peacock system in solid water. Films were placed in the solid water phantom in order to measure the dose distribution for comparison against the Monte Carlo calculations. Matlab functions were written to convert the Monte Carlo output into a format RIT113 (RIT Inc., Colorado Springs, CO) could read. This allowed us to co-register the calculated dose maps and the measured ones in order to compare the two. Results: The Monte Carlo calculated dose distribution from the complete arc therapy in solid water phantom was compared against film measurements. The agreement was within 2%. The comparison between Monte Carlo results and Corvus calculated dose distribution revealed that Corvus would fail to accurately compute the dose in the region where inhomogeneities were present. Conclusions: Based on the agreement between Monte Carlo and measurements we can use the Monte Carlo system as an independent quality assurance tool in order to verify dose distributions and MUs per arc computed by the Corvus.
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| 319. |
- Sturesson, C, et al.
(författare)
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Tissue temperature control using a water-cooled applicator: Implications for transurethral laser-induced thermotherapy of benign prostatic hyperplasia
- 1997
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 24:3, s. 461-470
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Tidskriftsartikel (refereegranskat)abstract
- A prototype to a water-cooled applicator to be used in transurethral laser-induced thermotherapy of benign prostatic hyperplasia was developed. The flexible applicator was made of Teflon(TM) tubes except for the distal outer part which was made of glass, providing a transparent medium for laser radiation and enabling efficient cooling of the surrounding tissue. For heating, laser light from a Nd:YAG laser emitting at 1064 nm, which was coupled into an optical fiber with an institutionally made diffusing tip, was used. Cooling was performed by flushing water through the applicator. By using a mathematical model it was possible to connect the temperature rise of the water in the applicator to the maximum tissue temperature. Tissue light absorption was calculated using Monte Carlo simulations and the heat conduction equation was solved numerically using a finite-difference technique. Experiments on porcine liver in vitro showed that the maximum tissue temperature could be estimated with an average accuracy of 0.4 degrees C by measuring the difference in outlet and inlet applicator water temperature and using the thermal model. The results presented suggest that the described method for temperature control can be used during laser prostatectomy to maximize the lesion size while preventing carbonization. (C) 1997 American Association of Physicists in Medicine.
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| 320. |
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